This invention relates generally to clamps for holding wafers during processing in deposition chambers. In particular, the invention relates to clamps that prevent bridges of deposition materials from forming between the wafer and the wafer clamp.
During fabrication of integrated circuits, chemical vapor deposition (CVD) is employed to deposit layers of certain materials such as polysilicon, tungsten, and silicon nitride on a semiconductor wafer substrate. In the CVD process, gaseous reactants chemically react to deposit individual atoms of a particular element or elements on the wafer surface. FIG. 1 illustrates a conventional CVD processing chamber generally referred to by the reference numeral 10. A hermetically sealed chamber 12 provides a contaminant-free environment for the deposition reaction. A gas box 14, located outside of chamber 12, supplies the gaseous chemical reactants at predefined flow rates and pressures to a shower head 16 located inside chamber 12. The shower head 16 serves to disperse the gaseous reactants evenly throughout the top of chamber 12. Once inside the chamber 12, the reactants are converted to a plasma in order to increase their reactivity. The active reactants then flow or diffuse throughout the reactor interior and ultimately react on the surface of wafer 18 to deposit a layer. During the CVD reaction, the wafer 18 is supported by a susceptor 20 (or other support) and secured to the susceptor by a wafer clamp 22.
Recently, it has been observed that in certain CVD reactors, such as the P-5000 available from Applied Materials, Inc. of Santa Clara, Calif., the deposition process sometimes produces a deposition layer that contacts and adheres to the wafer clamp 22 as well as the wafer 18. When this happens, part of the layer breaks off of the wafer when the wafer is unclamped. IC yield is thereby decreased in at least two ways. First, any ICs on the periphery of the wafer which have their associated deposition layer partially removed will be unusable. Second, particles of deposition material generated when the wafer is unclamped may settle on the wafer surface where they can destroy other ICs. These problems are illustrated in FIGS. 2a and 2b.
Referring first to FIG. 2a, a prior clamp-wafer-susceptor assembly 24 is shown with a deposited layer 26. Clamp 22 of assembly 24 is of circumferential configuration and applies a downward force normal to a top surface of wafer 18 so as to secure wafer 18 against the susceptor 20. Clamp 22 includes an overhang member 23 which has a rounded edge and overlies a peripheral region of the semiconductor wafer--i.e., approximately the outer 5 mm of the wafer top surface. While the goal of CVD is to form a layer on an active area of wafer 18 (i.e., the area where the ICs are formed), the gaseous reactants diffuse throughout reactor chamber and deposit material on other surfaces as well. Further, some reactant molecules/radicals move in horizontal as well as vertical directions, thereby finding their way into crevices or cavities where they react and deposit solid material. For example, the layer 26 on wafer 18 can extend slightly under the rounded edges of overhang members 23 to form a buildup or bridge of deposited material 28 between clamp 22 and the wafer 18. As noted, when the clamp is lifted up to release the wafer 18, some of layer 26 will break away and form flakes which may land on the wafer surface. Further, the layer 26 may break away at some peripheral ICs, thereby destroying them. Specifically, in the case of tungsten deposition to fill vias, when the overlying tungsten layer is partially broken off by unclamping, the tungsten in the underlying vias is exposed and etched away during a subsequent etch back step. Thus, the amount of tungsten in the via will be inadequate to provide electrical contact between vertically separated layers.
FIG. 2b illustrates the problem of the flakes settling on the wafer surface. After tungsten (or other material) deposition is complete, the wafer 18 is unclamped from clamp 22 for further processing. As shown, small tungsten flakes 30 may break off from the bridges 28 (shown in FIG. 2a) during unclamping and then fall onto the newly deposited layer 26. These flakes can destroy ICs in various ways. For example, they may block complete etch back of tungsten layer 26 in subsequent steps, thereby providing an unintended conductive path or short between metallization lines. This is because the etch back process removes the same amount of tungsten regardless of variations in tungsten layer thickness (caused by the flakes for example). Thus, the etch does not planarize the wafer surface, and flakes 30 constitute, in effect, micromasks which produce metal images of themselves on the underlying wafer surface following etch back. Such residual metal images may contact a subsequent deposited metal layer thereby shorting it to the metal layer below.
It should be noted that the problem of bridging can occur with many different depostion materials. However, it very frequently occurs when depositing tungsten because tungsten formed from WF.sub.6 and SiH.sub.4 tends to deposit in small crevices or cavities. While this property makes tungsten a good choice for filling vias to make contacts, it unfortunately also promotes buildups such as bridge 28.
In view of the above, what is needed is a clamp design that reduces the likelihood of bridging material forming between the clamp and wafer during deposition.